1. Field of the Invention
The present invention relates to an optical fiber and to an optical transmission system employing this optical fiber. More specifically, the present invention relates to an optical fiber having a relatively simple structure, that has an increased effective core area and can reduce nonlinearity in an employed wavelength band selected from the range of 1.53˜1.63 μm, and to an optical transmission system employing this optical fiber.
This application is based on patent applications filed in Japan (Japanese Patent Application No. Hei 11-228904 and No. 2000-12259), the contents of which are incorporated herein by reference.
2. Background Art
As erbium-doped optical fiber amplifiers have entered practical use, systems such as very-long-distance non-repeating relays for wavelengths of 1.53˜1.63 μm have become commercially available. Further, the development of WDM (wavelength division multiplex) transmission has progressed rapidly with the increase in transmission capacity, with wavelength multiplex transmission already commercially available on a number of transmission paths. Rapid growth in the number of wavelength multiplexes is anticipated in the future.
In WDM transmission, the optical power propagating through the optical fiber is very greatly increased in order to transmit a number of optical signals of different wavelengths in a single optical fiber. For this purpose, it is essential to have a technique for reducing nonlinearity so that deterioration in the transmission characteristics can be avoided.
The size of the nonlinearity is expressed as n2/Aeff, where n2 is the non-linear refractive index for the optical fiber, and Aeff is the effective core area of the optical fiber. In order to decrease the nonlinearity, n2 must be decreased or Aeff must be increased.
Because n2 is a constant value for the material, it is very difficult to significantly decrease n2 in a quartz-derived optical fiber. Accordingly, Aeff must be increased. A number of designs having complicated refractive index profiles have been developed, however, all of these are quite expensive.
On the other hand, 1.3 μm single mode optical fibers have been widely used in conventional transmission systems.
However, when the employed wavelength band is set in the 1.53˜1.63 μm range in the case of a 1.3 μm single mode optical fiber, then bending loss (macrobending loss) increases and there is a worsening of the transmission loss that arises due to the slight bending that occurs during laying of the optical fiber.
A cut-off shift optical fiber (CSF) has therefore been proposed in which the bending loss is reduced by making improvements to the 1.3 μm single mode optical fiber.
The CSF has low bending loss in 1.5 μm wavelength band by shifting conventional cut-off wavelength (1.2 μm) of the 1.3 μm single mode optical fiber into 1.4˜1.5 μm.
However, since this cut-off shift optical fiber is not intended to control nonlinearity, it does not have an Aeff that is large enough for this purpose.